I found several years ago that my '86 Vanagon with a weak battery would
not start (although the engine would turn over just fine), if the
voltage, measured between the battery posts, dropped below 10.0 volts
while attempting to crank the engine.  If the voltage remained above
10.0 volts the engine would start fine.  I have duplicated this test on
two occasions.  A new battery always fixed this problem.

I would imagine that if the voltage at the battery is 10.0 volts, the
voltage available to the digifant system (and ignition coil) would be
much less.

This is an easy thing to measure.